Conventionally, automobile parts, construction machine parts and the like are produced from a hot-rolled steel wire rods and bars through softening annealing to ensure cold workability, forming by cold working such as drawing and cold forging, and hardening and tempering. In the softening annealing step, for example, in the case of producing a bolt, as a machine part, from a hot-rolled wire, low-temperature annealing at about 650° C. for 2 hours is applied for stud bolt, and the like, with a small cold working amount, normal annealing at about 700° C. for 3 hours is applied for hexagon bolt and the like, and spheroidizing annealing at about 720° C. for 20 hours is applied for bolt with flange, and the like, with a large cold working amount, thereby ensuring cold workability. These softening annealing steps take a long time and moreover, the cost for annealing occupies a large portion of the cost for production of machine parts and the like due to a recent rise in energy price. Therefore, from the standpoint of improving the productivity and saving energy, various techniques have been proposed to avoid the softening annealing before the cold working. For example, there have been proposed a production method for a low alloy steel having excellent cold workability in Japanese Unexamined Patent Publication No. 57-73123, a direct softening method of steel wire rods and bars for structural use in Japanese Unexamined Patent Publication No. 58-58235, a production method of a directly softened wire and rod in Japanese Unexamined Patent Publication No. 2-185920, and a production method of a machine structure steel suitable for cold working in Japanese Unexamined Patent Publication No. 8-209236.
However, the as-hot-rolled steel wire rods and bars obtained by these production methods is insufficient in the cold workability as compared with conventional steel wire rods and bars subjected to softening annealing. At present, a soft steel wire rods and bars for machine structural use, which can be satisfactorily used in practice in the as-hot-rolled state, has not been attained.
The present inventors have studied these problems and have proposed a steel softened comparably with an annealed steel in Japanese Patent Application No. 11-146625. However, a steel capable of providing cold workability greater than conventional softened and annealed steels, even in the case of a large working degree, is still required.